The present invention is related generally to radio frequency (RF) electronic filters, and more particularly to an improved adjustable ceramic filter and method of tuning that is particularly well adapted for use in radio transmitting and receiving equipment.
Many structures for multi-resonator filters are known. One such structure includes ceramic filters comprised of a dielectric block having one or more holes extending from its top surface to its bottom surface and further having first and second electrodes each disposed on the dielectric block at a pre-determined distance from a corresponding hole. If there is only one hole in the dielectric block, the first and second electrodes may be arranged around that hole. If there are two or more holes in the dielectric block, the first electrode may be located near the hole at one end of the dielectric block and the second electrode may be located near the hole at the opposite end of the dielectric block. A conformal conductive coating, or plating, covers essentially the entire surface of the dielectric block, including each through-hole, for forming a transmission line which has an open portion in the plating to provide a resonator by including capacitive reactance thereat and except for portions near included first and second electrodes. Coupling between resonators is controlled by included conductive slots between resonators, or merely by the spacing between resonators being set to a predetermined distance.
Each resonator is generally set at a frequency lower than a desired frequency, and then subsequently tuned by removing capacitive portions of the conductive coating from each resonator in a pre-established tuning sequence, usually accomplished by the removal of additional ground plating near the top, capacitive region, of each plated hole while monitoring the return loss angle of the filter. This tuning process is implemented by initially grounding the plating at the top of each plated hole and then measuring an initial value of the return loss angle. Then, with the ground to each plated hole removed one at a time, the ground plating near the top of that plated hole is trimmed or selectively removed, until a phase target of 180 degrees of phase shift is achieved. The ground provided to each plated through-hole can be done manually by means of a metallic instrument, or by means of including a small plating runner that bridges the unplated area, or capacitive region, between the plated through-hole and the surrounding conformal plating on the dielectric block.
However, due to the subtractive tuning process just described, the above structure and tuning method suffers from several serious drawbacks. The first is that removal of relatively small selected portions of the conductive coating near the top of each resonator can cause relatively large upward frequency shifts. Thus, if too much conductive material is removed, the phase target can be missed and the resonator will be tuned at a frequency much higher than the desired resonant frequency. The second drawback is that such a tuning method only provides uni-directional tuning.
Although one known method of restoring an overtuned resonator back down to its desired resonant frequency includes the use of conductive paint, such a process consumes additional time and must be done carefully to insure that the resonator ultimately operates at its desired frequency. The additional steps involved in utilizing such a method are to be avoided, particularly when constructing and tuning large volumes of such ceramic filters. Clearly, what is needed is a new method of tuning, utilizing the preferred subtractive process, such that bi-directional tuning is possible. This new method should, therefore, be able to provide real-time, on-line adjustment of one or more resonators in a ceramic filter, in order to reduce overall production time and cost incurred during construction and tuning.